Formation of a barrier to radial oxygen loss in L-type lateral roots of rice

A barrier to restrict radial oxygen loss (ROL) promotes the adaptation of plants to waterlogged soil conditions. A ROL barrier is formed in the basal parts of roots and contributes to the enhancement of the longitudinal diffusion of oxygen (O2) via aerenchyma toward the root tips. The adventitious roots of rice (Oryza sativa) can form a ROL barrier in waterlogged soil; however, whether a ROL barrier can be formed in the lateral roots (LRs) of rice remains unclear. Rice possesses two types of LRs: L- and S-type. L-type LRs are generally long, thick, and capable of branching, whereas S-type LRs are short, thin, and incapable of branching. In this study, we examined whether the L- and S-type LRs of rice possess the ability to form ROL barriers. In L-type LRs, a ROL barrier was formed; the aerenchyma, which is constitutively formed under aerated conditions, was further developed under stagnant deoxygenated conditions, suggesting that these traits promote the supply of O2 to the root tips of LRs. However, neither a ROL barrier, nor aerenchyma was formed in S-type LRs, and thus ROL was observed mostly in the basal parts. Patterns of deposition of suberin, which is thought to be one of the components of the ROL barrier, were consistent with the patterns of ROL-barrier formation in L-type and S-type LRs. These results suggest that L- and S-type LRs play distinct roles in the growth of rice plants in waterlogged soils and in oxygenating the rhizosphere.